RFC 5321 section 4.5.2 SMTP Transparency (aka "dot escaping")
val encode_line_string : string ‑> Email_message.String_monoid.t
encode_line_*
and decode_line_*
dot escape and un-dot escape, respectively, a
given line. It does NOT handle new line breaks in the middle of the supplied strings.
val encode_line_bigstring : Core.Bigstring.t ‑> Email_message.String_monoid.t
val decode_line_bigstring : Core.Bigstring.t ‑> Core.Bigstring.t